Shares of ValueClick ( VCLK) tumbled Tuesday after the company said sales would be lower than expected for both the third quarter and the full year. The online advertising service company attributed the shortfall to weakness in its lead generation business. ValueClick was recently off 12.1% to $24.42. Shares had gained more than 40% over the last month amid an increasingly bullish outlook for the sector and speculation that the company could be the target of a takeover. For the third quarter, ValueClick now expects sales of $156 million to $157 million, down from prior guidance of $155 million to $165 million. The company reaffirmed third-quarter earnings guidance of 16 cents to 17 cents a share. Analysts surveyed by Thomson Financial had expected earnings of 17 cents on revenue of $159 million. For the full year, the company now expects sales of between $635 million and $640 million, down from earlier projections of $645 million to $660 million. Analysts were expecting $650 million.